Performance-based multi-objective optimization of a direction-decoupled three-dimensional base isolation system

Vertical seismic effects are often overlooked but can amplify vertical displacements and accelerations and may trigger vertical instability. Conventional isolators, while designed to carry gravity loads, typically possess high vertical stiffness, which limits vertical isolation effectiveness under strong shaking. In direction-decoupled three-dimensional isolation, coordinated control of horizontal and vertical performance is therefore essential, and optimization provides a systematic route to achieve this balance. This study develops a performance-oriented, multi-objective parameter-optimization framework for a direction-decoupled three-dimensional base-isolation system (3D-DIS) comprising a laminated rubber bearing (LRB) in series with a ring-spring vertical isolator with a central ring (RSVIB-CR). Under three-component ground motions, nonlinear time-history analysis (NTHA) is coupled with NSGA-II to jointly minimize, within device-feasible bounds, the maximum interstory drift ratio (IDR), the peak floor vertical acceleration (Az), and the maximum vertical displacement of the isolation layer (Diso). Seven ground-motion records are used for optimization and fourteen additional records for post-optimization validation. Relative to a fixed-base structure, the optimized design achieves median reductions of ∼60% in peak roof horizontal acceleration and 35–43% in peak interstory drift across pulse-like near-fault, non-pulse near-fault, and far-field motions, while keeping the vertical isolator stroke within prescribed limits. A LOESS-based sensitivity analysis identifies robust parameter ranges, providing actionable guidance for practice.